1. FIELD OF THE INVENTION
This invention relates to a process for the production of polyesters from an aromatic dicarboxylic acid component and 1,4-butanediol. More particularly, this invention is directed to the preparation of a linear polyester wherein the quantity of 1,4-butanediol employed in the polymerization is substantially smaller than the quantities heretofore employed. The process of the present invention overcomes some of the known drawbacks connected with prior art polymerization processes.
2. DISCUSSION OF THE PRIOR ART
The use of 1,4-butanediol for the production of polyesters has been known for a long time. The dicarboxylic acid condensed-in may be both an aliphatic and an aromatic dicarboxylic acid. In the event that the polyesters contain condensed therein only aliphatic dicarboxylic acids in addition to 1,4-butanediol, products are obtained which, in practice, have not gained any major importance since their melting and/or softening points are extremely low. On the other hand, polyesters of 1,4-butanediol and aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, etc., possibly in admixture with aliphatic dicarboxylic acids such as adipic acid, sebacic acid, dimeric fatty acids, etc., are of economic importance, e.g., as fusion bonding agents, as raw materials for the production of coatings according to the known powder coating processes, for the production of rubber-elastic block polycondensates. Some polyesters of this type are described, for instance, in the French Pat. No. 1,441,060 or in the U.S. Pat. No. 3,423,281.
Thus far, these polyesters have generally been made in two stages, analogous to the processes that are customarily applied for the production of polyethylene terephthalate. In these processes, the mixture of the aromatic dicarboxylic acids and/or lower alkyl esters of these dicarboxylic acids is first reacted in the melt at temperatures above 200.degree. C with at least a 50% molar excess of 1,4-butanediol and then the excess 1,4-butanediol is removed by distillation by means of further temperature increase and subsequent application of a vacuum until the molar ratio of the radicals of the dicarboxylic acids to those of the 1,4-butanediol in the reaction product has become approximately 1:1. This molar ratio means that a polyester of high molecular weight has been produced.
But this process has the following drawbacks: At the high temperatures prevailing during condensation considerable quantities of tetrahydrofuran and water are continuously produced, which have to be distilled off as well. Therefore, the distiller-off 1,4-butanediol is contaminated and cannot be reused directly. The working-up of the 1,4-butanediol/tetrahydrofuran/water mixture by distillation is very difficult and large amounts of 1,4-butanediol are lost.
It is therefore of considerable economic significance to modify the process outlined above for the production of polyesters containing condensed therein 1,4-butanediol in such a way that the aforementioned drawbacks do not arise. The obvious solution, i.e., to reduce the excess of 1,4-butanediol used, entails the following difficulties:
As is well-known, it is impossible in the analogous production of polyethylene terephthalate to use a quantity of less than approximately 1.5 moles of ethylene glycol to 1 mole of dialkyl terephthalate while maintaining the usual catalyst concentrations (ranging between 10.sup.-4 and 10.sup.-4 moles/mole of ester) if the ester-interchange reaction is to proceed to an extent that polyesters of sufficiently high molecular weight are obtained in a reproducible manner in the subsequent polycondensation reaction. If the above specified catalyst concentration is increased to such a marked degree that the quantity of ethylene glycol used can be reduced further with respect to a reproducible polycondensability of the starting materials, technically worthless polyesters are obtained, containing large amounts of cross-linked constituents, having low thermal stabilities and showing, as a rule, strong discolorations.
A process has been described for the production of polytetramethylene sebacate by ester-interchange between 1 mole of dimethyl sebacate and only 1.1 mol of 1,4-butanediol, in which temperatures between 172.degree. and 215.degree. C are used throughout and a vacuum is applied (C. S. Marvel, J. H. Johnson, J. Am. Chem. Soc. 72 (1950), 1674). However, with 9 .multidot. 10.sup.-1 mole/mole of ester the catalyst concentration is higher by 1 to 3 powers of ten than customary, which, on the basis of the above, must result in extensive thermal decompositions and cross-linkings at higher temperatures than those used of max. 215.degree. C. In those cases in which the polycondensate contains condensed therein aromatic dicarboxylic acids as well, temperatures around only 215.degree. C cannot be maintained either because of the melting point of more than 215.degree. C of the polyester to be produced or as a result of too high a melt viscosity so that this method can be used only specifically for the production of polytetramethylene sebacate and cannot be applied to the production of polyesters containing aromatic dicarboxylic acids condensed therein.